Epidemiology and Antimicrobial Susceptibility of Slow-Growing Non-Tuberculous Mycobacteria in a TB-Endemic Setting: An 8-Year Laboratory-Based Study from North India
Background: Differentiating non-tuberculous mycobacteria (NTM) from Mycobacterium tuberculosis remains a significant diagnostic challenge in tuberculosis (TB)-endemic settings, often leading to misdiagnosis and inappropriate therapy. Data on the epidemiology and antimicrobial susceptibility of slow-growing mycobacteria (SGM) in North India remain limited. This study aimed to characterize the prevalence, species distribution, and in vitro antimicrobial susceptibility patterns of SGM isolated from clinical specimens in a TB-burden setting. Methods: A retrospective laboratory-based study was conducted on clinical specimens processed between January 2018 and December 2025 at a tertiary reference laboratory in Delhi, India. Mycobacterial cultures were performed using liquid (MGIT 960) and solid (Lowenstein-Jensen) media. Species identification was carried out using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Antimicrobial susceptibility testing (AST) was performed on a subset of isolates using broth microdilution in accordance with Clinical and Laboratory Standards Institute (CLSI) guidelines. Temporal trends were assessed using the Cochran-Armitage test and Joinpoint regression analysis. Results: Of 197,943 clinical specimens processed, 34,776 (17.5%) yielded mycobacterial growth; of these, 30,343 (87.3%) were identified as members of the Mycobacterium tuberculosis complex, while 4433 (12.7%) were classified as NTM. Among these, 1845 (41.6%) were slow-growing mycobacteria (SGM) species. Pulmonary specimens accounted for the majority of SGM isolates (63.2%). The Mycobacterium avium complex (MAC) predominated, with M. chimaera representing 51.7% of SGM isolates, followed by M. simiae (13.0%) and M. kansasii (12.7%). Non-chromogenic species constitute 67.8% of isolates. Antimicrobial susceptibility testing (n = 204) demonstrated high in vitro activity of amikacin (99.5%) and clarithromycin (97.5%), followed by linezolid (95.2%) and moxifloxacin (94.6%), whereas comparatively higher resistance rates were observed for doxycycline, ciprofloxacin, and minocycline. Temporal analysis showed year-to-year variability without a statistically significant trend (annual percent change +0.15%; p = 0.963), indicating a stable proportional burden of NTM over time. Conclusions: This laboratory-based study highlights the persistent and diverse presence of slow-growing NTM in a TB-endemic setting, with MAC predominance across pulmonary and extrapulmonary specimens.
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